Conventionally, an X-ray CT apparatus which irradiates a subject with X-rays, detects X-rays transmitted through or scattered by the subject using an X-ray detector, and forms a fluoroscopic image, tomosynthesis, or three-dimensional (3D) image on the basis of this X-ray detection output (the number of photons of X-rays) is known. As such X-ray CT apparatus, a cone beam CT apparatus has been developed. In a normal X-ray CT apparatus, an X-ray beam is limited in the Z-direction, and is called a fan beam. However, cone beam CT (CBCT) uses an X-ray beam which also spreads in the Z-direction, and for this reason is called a cone beam.
In CT, a half-scanning technique that scans through “180°+fan angle” is known. In this half-scanning technique, when a gantry which mounts an X-ray generation source and X-ray detector is not located at a predetermined rotation position (one of 0°, 90°, 180°, and 270°), the control waits until the gantry reaches this predetermined rotation position, and then starts measurement. For this reason, the actual measurement start time is indefinite and typically does not coincide with a time planned for starting measurement, resulting in poor time-setting precision. That is, in order to obtain a tomosynthesis for a subject to be examined, the subject is inserted into an opening of the gantry, which is rotated around the subject through 360°, and measurement (imaging) starts from a predetermined measurement start angle (for example, one of 0°, 90°, 180°, and 270° detected by an angle detector). An image processing apparatus performs image reconstruction required to obtain a tomosynthesis using measurement data obtained in this way. The image reconstruction is done under the condition that data measured by a scanner starts from the predetermined angle (an angle as one of 0°, 90°, 180°, and 270°).
This “start” is an initial 0° equivalent position in the sense that the image reconstruction in the half-scanning method reconstructs an image using projection data for what is thought of as 0° to 180°. This 0° equivalent position, as described above, is actually one of four absolute angles, i.e., 0°, 90°, 180°, and 270°, as the absolute angle of orientation of the gantry.
A technique for implementing such process is proposed by Japanese Patent No. 03347765 (to be referred to as reference 1 hereinafter). According to reference 1, measurement data appended with the detection angle of the gantry is obtained, and a data start angle position (0° to 360° or 0° to 180°) and data for 360° or 180° turn from this start angle position can be determined from the angle appended to the measurement data. In this way, the measurement start position can be freely set at an angle other than 0°, 90°, 180°, and 270°. Furthermore, even when an angle θ which does not reach each of the 0°, 90°, 180°, and 270° positions (0°<θ<90°, 90°<θ<180°, 180°<θ<270°, 270°<θ<360°) is set, that measurement start position can be set as a measurement start position of data for 360° or 180° turn as long as the measurement starts.
On the other hand, Japanese Patent Laid-Open No. 2001-224588 (to be referred to as reference 2 hereinafter) discloses a technique for imaging a body part (heart) of a patient having cyclic motions using a slice imaging system in association with heart imaging using half-scanning. An axial “half-scan” is segmented into N sectors (N being a positive integer which is equal to or larger than 2). Note that “half-scanning” involves executing a scan over a view angle range equal to an angle obtained by adding one fan angle to 180°. According to reference 2, image data that represent at least one half-scan is acquired by acquiring image data corresponding to each of N sectors in a corresponding heartbeat cycle of the patient for at least N heartbeat cycles. In this known technique, only one sector is acquired per heartbeat cycle. When the imaging system is a multi-slice imaging system, only one sector for each slice is acquired per heartbeat cycle. Since sectors acquired during one heartbeat cycle are acquired in a relatively short period of time in substantially the same parts of these heartbeat cycles, the sectors being combined are ones which have undergone short scans and are obtained from such parts in different heartbeat cycles, thus reducing motion artifacts in the final image. Note that “substantially the same parts of the heartbeat cycles” means that since the positions of a heart in these parts of the heartbeat cycles are similar and have no difference, a reconstructed image important for diagnosis and medical purpose is free from deterioration independently of the difference in position of the heart. A step of gate-driving a radiation source and a step of acquiring sectors of image data are repeated until image data that represent at least a half-scan of one image slice are acquired.
Furthermore, according to Japanese Patent Laid-Open No. 2002-355241 (to be referred to as reference 3 hereinafter), in imaging using half-scanning, an artifact resulting from the motion of a subject is introduced as a maximum mismatch level present in neighboring projection views in CT data set. For example, full scanning typically assumes that a mismatch from the start to the end of scanning is the worst case possible. Upon scanning a subject making recursive motions (which are not always truly cyclic), when the subject has approximately the same motion states at the beginning and the end of scanning, a motion artifact is minimized. That is, it is known that a motion artifact is minimized when the motion cycle precisely matches the cycle of the gantry speed with respect to half-scanning and full scanning. In order to minimize a motion artifact, it is proposed to determine a start projection view after a plurality of projection views are acquired. That is, a difference between the first and the last views used in reconstruction is determined, and a view that minimizes the difference is selected as the start view. For example, the start angle of half-scanning that minimizes motion-induced artifacts is determined by difference projection.
The techniques proposed by references 1 to 3 above reduce generation of artifacts due to subject motions and patient heartbeats by determining data corresponding to an appropriate half-scan from data for one rotation obtained by X-ray imaging and using the determined data in reconstruction. For this purpose, the gantry must make one or more rotations to obtain half-scan data. On the other hand, since the imaging time is preferably short in terms of reducing the burden on a patient and reducing the influence of body motion errors, a measure for increasing the rotation speed of the gantry must be taken.
On the other hand, in CBCT, when a large organ such as lungs or the like is to be imaged by a single scan, since the cone angle is limited to a small value so as not to cause reconstruction errors, the FDD (focus detector distance) must be set to be as large as about 2.5 m. In this case, the following problems are posed:
(1) The gantry becomes bulky, and cannot be carried into a room in the case of a recumbent position CT which is currently distributed. Since a large centrifugal force is generated upon rotation, a high-speed imaging system cannot be adopted.
(2) When a type that rotates a subject (subject rotation type) is adopted, the scan time ranges from 5 to 10 sec/rotation.
Especially, in the subject rotation type, use of half-scanning with a shorter scanning time is strongly demanded so as to reduce errors caused by body motion. In order to meet a requirement of reducing the patient's exposure to X-rays, a technique for executing half-scan imaging itself at a suited timing is required in place of the conventional method that selects half-scan data from one-rotation data.